Fix use-after-free bug in RemoteDataSource class

"getSize()", "getIMemory()", "readAt()" and so on can be called after
mSource/mMemory is released from other thread, which can result in
use-after-free problem.
Solve by adding lock and check before using/changing.

Bug: 151642837
Test: run poc in bug report.
Change-Id: Id37a6bfc38e290a67575afc31c112d7095362335